1. Field of the Invention
The present invention relates to an organic electrolyte and a lithium secondary battery, and more particularly, to an organic electrolyte which provides a lithium secondary battery having improved charge/discharge cycle characteristics while exhibiting excellent discharge capacity and low-temperature discharge characteristics, and a lithium secondary battery employing the same.
2. Description of the Related Art
In recent years, electronic equipment has rapidly become smaller and lighter. In particular, in the fields of office automation, lap-top computers, notebook computers or the like have been substituted for desk-top computers. Also, portable electronic equipment such as camcorders, cellular phones or PCS phones have been rapidly in widespread.
With rapid development of smaller and lighter electronic equipment, highly efficient secondary batteries for use as electric sources have been in high demand. In other words, there has been rapid development of lithium secondary batteries which can be a substitute for conventional lead-storage batteries or nickel-cadmium batteries, have a high energy density for fulfilling the requirements of a small and light battery and are capable of being repeatedly charged and discharged.
A lithium secondary battery includes a cathode and an anode using active materials which allow intercalation and deintercalation of lithium ions, and an organic electrolyte or a polymer electrolyte through which lithium ions can be moved is filled between the cathode and the anode, and electrical energy is generated by oxidation and reduction carried out when intercalation/deintercalation of lithium ions occur at the cathode and the anode.
As anode active materials of lithium secondary batteries, metallic lithium capable of reversibly occluding or releasing lithium ions while maintaining structural and electrical properties, lithium alloys or carbon materials having substantially the same chemical potential of lithium intercalation/deintercalation as the metallic lithium, are mainly used.
Lithium secondary batteries using metallic lithium or lithium alloys are referred to as lithium metal batteries, and those using carbon materials are referred to as lithium ion batteries. The lithium metal batteries which are liable to explosion due to short-circuiting caused by growth of dendrites are being replaced with lithium ion batteries using carbon materials free of a danger of explosion as the anode active materials. In the lithium ion batteries, during charging and discharging, only lithium ions are mobile and active materials are retained without being changed. Thus, compared to the lithium metal batteries, the lithium ion batteries exhibit improved cycle life and safety.
As cathode active materials of lithium secondary batteries, complex oxides of transition metals and lithium, which show a voltage 3 to 4.5 V higher than that of Li/Li+ and are capable of intercalation/deintercalation of lithium ions, are mainly used. Examples of the cathode active materials include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2 or LiMn2O4), lithium nickel cobalt oxide (LiNi1-xCoxO2) and the like. Manganese-based materials such as LiMnO2 or LiMn2O4 are attractive materials because they are easily prepared and relatively cheap and cause little environmental pollution. However, the manganese-based materials still have a problem of small capacity. Although lithium cobalt oxides (LiCoO2) exhibiting a high electrical conductivity, a high battery voltage and excellent electrode characteristics are typically used cathode active materials which are commercially available from Sony Corporation of Japan, they are expensive. Among the above-described cathode active materials, lithium nickel oxides (LiNiO2) are relatively cheap and exhibit the highest discharge capacity. However, they are difficult to prepare and are poor in battery safety due to their high discharge capacity.
Since a lithium secondary battery exhibits its battery characteristics by complex reactions between cathode/electrolyte, anode/electrolyte and so on, the use of an appropriate organic electrolyte is one of important factors for improving the performance of the lithium secondary battery. An organic electrolyte is an ion-conductor produced by dissolving a lithium salt in an organic solvent and should be excellent in lithium ion conductivity and chemical and electrochemical stabilities for electrodes. Also, the organic electrolyte should be capable of being used over a broad range of temperature and the manufacturing cost thereof should be low. Thus, it is preferred to use an organic solvent having a low viscosity while having high ion conductivity and a high dielectric constant.
However, there is no single organic solvent which meets these requirements. Thus, the composition of an organic solvent contained in an organic electrolyte is generally a 2-component mixture of a high dielectric constant solvent and a low viscosity solvent (see U.S. Pat. No. 5,437,945 and U.S. Pat. No. 5,639,575), or a 3-component mixture further comprising a third organic solvent having a low freezing point (see U.S. Pat. No. 5,474,862 and U.S. Pat. No. 5,639,575).
The use of such a mixed organic solvent increases mobility of lithium ions, thereby improving ion conductivity and initial discharge capacity of a battery. However, as the cycles proceed, the organic electrolyte undergoes a surface reaction with graphite, which is an anode active material, to degrade discharge capacity, resulting in poor charge/discharge cycle characteristics.
To solve the above problems, it is an object of the present invention to provide an organic electrolyte which does not readily react with an anode active material, thereby improving charge/discharge cycle characteristics of a lithium secondary battery.
It is another object of the present invention to provide a lithium secondary battery having improved charge/discharge cycle characteristics by using the organic electrolyte.
Accordingly, to achieve the first object, there is provided an organic electrolyte containing a mixed organic solvent and a lithium salt, wherein the mixed organic solvent comprises 20 to 60% by volume of ethylene carbonate, 5 to 30% by volume of propylene carbonate and 20 to 70% by volume of chain carbonate.
In the organic electrolyte, the content of a mixture of ethylene carbonate and propylene carbonate is preferably 25 to 65% by volume.
Also, the chain carbonate is preferably at least one compound selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylethyl carbonate and methylpropyl carbonate.
Further, the lithium salt is preferably at least one compound selected from the group consisting of LiPF6, LiBF4, LiAsF6, LiClO4, CF3SO3Li, LiC(CF3SO2)3, LiN(C2F5SO2)2 and LiN(CF3SO2)2.
According to another aspect of the present invention, there is provided a lithium secondary battery including a cathode having lithium containing metal oxide, an anode having metallic lithium, a lithium alloy or a carbon material, and the organic electrolyte according to the first aspect of the present invention.